1. Field of the Invention
The present invention relates to a magnetic reproduce head, and in particular to a magnetoresistive reproduce head employing materials exhibiting giant magnetoresistance.
2Description of the Prior Art
As magnetic recording technology continues to push areal recording density limits, magnetoresistive (MR) reproduce heads appear to be the technology of choice. Recent U.S. Pat. Nos. 5,084,794 and 5,193,038 (incorporated herein by reference) disclose dual magnetoresistive (DMR) reproduce heads which offer improved high linear density performance compared to conventional shielded magnetoresistive (SMR) heads, as well more robust operation and simpler fabrication. Until very recently, virtually all past magnetoresistive sensor/heads, including the DMR design, have been based on the physical phenomenon of anisotropic magnetoresistance (AMR) in Permalloy (NiFe) thin films.
The last few years have seen increased interest and research in the area known as "giant magnetoresistance" or GMR, which is a "giant" .DELTA.R/R response that is markedly greater in magnitude than that obtained by the AMR effect and is described below. Films observed to exhibit giant magnetoresistance can consist of multilayers of very thin (roughly 20 Angstrom) ferromagnetic films alternating with similarly thin layers of nonmagnetic conducting films typically of copper, silver, or gold.
The great potential of GMR films as magnetoresistive heads stems from the very large maximum change in resistance (.DELTA.R/R as much as 10-20% in some systems) that they can exhibit in response to magnetic fields, as compared to the .DELTA.R/R .perspectiveto.2% typical of traditional MR films employing the usual anisotropic magnetoresistance (AMR) effect. In general, GMR films are in a high resistance state when the magnetization in the GMR multilayers are predominantly antiparallel in consecutive magnetic layers, and can be then brought to a low resistance state by the action of an applied field which rotates each layer's magnetization into a predominantly parallel orientation roughly along the applied field direction.
Excluding the special case of the trilayer "spin-valve" design with an additional exchange pinning layer as taught in U.S. Pat. Nos. 5,159,513 and 5,206,590, or the spin-valve-DMR taught in the above-reference Ser. No. 08/208,755 application, multilayers which exhibit GMR in response to external applied fields generally require some intrinsic mechanism (exchange or magnetostatic in origin) which produces an effective antiferromagnetic coupling between adjacent ferromagnetic layers in order to maintain the aforementioned antiparallel alignment in zero external field. The strength of this antiferromagnetic coupling is denoted in terms of the effective coupling field H.sub.afc, which in most GMR multilayers is usually larger in magnitude than any existing uniaxial anisotropy of strength H.sub.k.
The intrinsic resistance R vs. (hard axis) applied field H response curve of a generic GMR multilayer (excluding hysteresis, if any) is shown in FIG. 1. To obtain the optimal sensitivity dR/dH with maximum dynamic range, a GMR multilayer would be operated at the indicated bias point, requiring a bias field H.sub.b .apprxeq.(H.sub.afc +H.sub.k)/.sqroot.2.apprxeq.H.sub.afc /.sqroot.2 in order to rotate the magnetization angle by .apprxeq.45.degree. such that sin.theta..sub.b.apprxeq. 1/.sqroot.2. (For small, patterned GMR reproduce head elements, one should replace H.sub.k with H.sub.k +H.sub.d, where H.sub.d is the geometry dependent demagnetization field of the element, which can be quite large, e.g., 100-200 Oe.) Much recent work on GMR has been directed to the development of materials with low effective H.sub.afc and large .DELTA.R/R in order to maximize the intrinsic sensitivity .DELTA.R/R/H.sub.afc. To date, the largest value of the latter, with .DELTA.R/R of 4-5% and H.sub.afc of 5-10 Oe, was reported by Hylton et al., reported in SCIENCE, vol. 261, pp. 1021-1024, Aug. 20, 1993. However, this and other reported R vs. H wafer level measurements of intrinsic H.sub.afc and .DELTA.R/R are generally made at current densities which are minuscule compared to those that would exist in the actual micron sized GMR reproduce heads contemplated for the application of these GMR materials. FIG. 2 indicates the spatial distribution of the internal current field H.sub.j (x,y) generated in a thin film of total thickness, T, carrying a uniform current density J. To a good approximation, H.sub.j (x,y).perspectiveto.(2.pi./5)J.times.Oe, with J in amp/cm.sup.2.
The current field H.sub.j (x) is antisymmetric about the film center (x=0) , i.e., H.sub.j (-x)=-H.sub.j (x). This is of opposite symmetry to that of the uniform bias field H.sub.b of FIG. 1, or more generally, the symmetric component H.sub.sig-s (x,y)=[H.sub.sig (x,y)+H.sub.sig (-x,y)]/2 of any signal field H.sub.sig (x,y) which such a uniformly biased GMR multilayer would be designed to measure using the teachings and practices of the current art. By itself, the current field H.sub.j (x) will induce an antisymmetric bias angle distribution sin.theta. j(x).apprxeq.H.sub.j (x)/H.sub.afc, which, it is important to note, is unimpeded by the size of H.sub.d because the antisymmetric sin.theta..sub.j (x) averages to zero. When subsequently superimposed (possibly non-linearly) on the desired uniform bias distribution sin.theta..sub.b .apprxeq.1.sqroot.2, there will result an asymmetric .theta.-distribution which can be skewed through the thickness of the GMR multilayer. In cases where H.sub.j-max =H.sub.j (x=T/2).gtoreq.H.sub.afc, there will likely occur substantial distortion of the internal GMR bias distribution in which opposite sides through the multilayer thickness will be substantially under biased (.vertline.sin.theta..vertline.&lt;&lt;12) or saturated (.vertline..theta..vertline..apprxeq..pi./2). Such bias distortion cannot be compensated for by any additional source of uniform (symmetric) bias field.
For future ultra high storage density applications, with sub-micron track widths, conventional AMR heads and/or spin-valve GMR designs will likely need to operate at current densities J.gtoreq.10.sup.7 amp/cm.sup.2 to achieve adequate signal levels. Since reproduce signal scales directly with current density, GMR multilayer heads will need to operate at similar J if they are to take full advantage of their large intrinsic .DELTA.R/R. For a GMR multilayer of total thickness T=400 A, this corresponds to H.sub.j-max .gtoreq.25 Oe. It follows that such a GMR multilayer with H.sub.afc .ltoreq.25 Oe, will, when operated at such competitive current densities, be expected to suffer the aforementioned distortion of their bias magnetization distribution that will inevitably lead to a significant reduction in the usable .DELTA.R/R, as well as loss of linear dynamic range and accompanying increase in nonlinear distortion of the reproduce signal.
Excluding the aforementioned DMR design, conventional MR head designs using either single layer AMR films, GMR spin-valves, or (uniformly biased) GMR multilayers will all require magnetic shield structures with MR-shield gaps of order 1/10 micron in order to obtain adequate linear reproduce resolution for future ultra high density recording systems. In addition to imposing significant technological difficulties such as insuring adequate electrical insulation of the MR head from very closely spaced conductive magnetic shields, such shielding structures also limit design options for biasing the MR element. Regarding the uniformly biased GMR multilayer, the only likely feasible bias technique known in the present art involves sense current self-bias by the inclusion of additional biasing layers, e.g., current shunt and/or magnetically-soft-adjacent-layers. Even for convention AMR films with small anisotropy fields of H.sub.k .perspectiveto.4-5 Oe, it is well known that these adjacent-layer bias techniques will require ever increasing bias current densities of &gt;10.sup.7 amp/cm.sup.2 to achieve adequate bias levels as the MR element height is reduced to 1 micron or less to accompany similarly small future reproduce track widths. This can further exacerbate the detrimental effects of the additional antisymmetric self-bias component of GMR multilayers described above. Although the latter problem can be eased, as implied above, by development and use of GMR materials with H.sub. afc of order 30-100 Oe, this in turn will add to the level of required current density to achieve adequate (uniform) bias of the shielded GMR multilayer, which then exacerbates internal bias problem, and so forth. The presence of additional biasing layers will in general also lower signal levels due to their electrical shunting of the MR layer(s).
The desirability of achieving a uniform bias state of sin.theta..perspectiveto.1/2 in a GMR multilayer is that it is the optimum bias state for maximizing reproduce sensitivity/dynamic range for detecting the symmetric component of any signal field. A brief, limited summary of the difficulties in achieving such a bias state have been outlined above. However, an approximately antisymmetric bias state, such as the internal sense current self-biased state of a GMR multilayer, is similarly optimal for detecting the antisymmetric signal field component H.sub.sig-a (x,y)=[H.sub.sig (x,y)-H.sub.sig (-x,y)]/2. At high linear magnetic recording densities, both H.sub.sig-s and H.sub.sig-a are similar in amplitude, and either one may be detected for the reproduction of information recorded on a magnetic medium. The DMR design, as taught in the above-incorporated '794 and '038 patents, is an example of an antisymmetrically self-biased MR head which exploits an intrinsic sensitivity to H.sub.sig-a in order to be able to achieve, without magnetic shields, comparable high density signal levels and superior linear resolution to the most narrow gap shielded MR heads that may be practically fabricated.